In constructing fixed digital radio link connections, it is essential that the probability of the connection being broken remains sufficiently low. In addition to equipment reliability, this probability is dependent on phenomena in the radio path which affect the radio wave propagation. Such phenomena include:
1. normal free-space attenuation, PA1 2. attenuation caused by rain, slowly changing in frequency and time, PA1 3. splitting of the signal propagation path, on account of changes in the atmospheric refractive index or reflections caused by terrain obstructions, into two or more routes (multipath propagation) and frequency-selective attenuation caused by the interaction of said routes, and PA1 4. interference arriving at the receiver from other radio link hops or other equipment.
A radio link hop is implemented in such a way that the fade margin for reception (that is, the ratio of the power of the desired signal to the sensitivity of the receiver in an operational situation in which interference is experienced) is sufficiently high. In such a situation, the probability that additional signal attenuation on account of phenomena 2 and 3 and deterioration of the reception sensitivity of the receiver on account of phenomenon 4 alone or in combination should be capable of cutting off the signal is sufficiently low. The signal is deemed as cut off when its bit error ratio exceeds a given limit value. This limit value is usually 10.sup.-3, which means a situation where every thousandth received bit, on an average, is erroneous. The limit value for the bit error ratio is termed the error ratio threshold and the corresponding input power value is termed the threshold power in the present context.
In conjunction with the commissioning of a radio link, the fade margin is usually checked under normal operational conditions. This check is conventionally made in such a way that the transmitter signal is attenuated until the bit error ratio of the receiver at the other end of the radio link hop deteriorates to the point of cut-off. The magnitude of attenuation gives directly the fade margin of the radio link hop. It should be noted that the fade margin cannot be directly calculated from the received signal power and the typical sensitivity of the receiver or the sensitivity measured in the manufacturing stage, as the effect of interference arriving at the receiver in an operational situation will then remain unobserved.
In the fade margin measurement carried out at the commissioning phase, attenuation of the transmission signal can be effected either in such a way that the transmission power of the actual transmitter can be adjusted or in such a way that a separate adjustable attenuator is connected after the transmitter for the time of measurement. The requisite attenuation is usually very high (30 . . . 50 dB), and the accuracy of it determines the accuracy of the measurement. Providing sufficiently accurate and wide-ranging power control in the actual transmitter will increase its cost and is very seldom economically feasible. On the other hand, using a separate attenuator is very cumbersome, especially when the radio parts of the radio link are located outdoors on an antenna mast.
Neither can such measurement be carried out by remote control, and hence the measurement must be carried out anew on site when it is suspected that the conditions have changed from those prevalent at the commissioning phase of the link.